In enhanced mobility MOSFET device applications thick, relaxed Si1-xGex buffer layers on bulk silicon wafers have been used as virtual substrates for thin strained silicon layers to increase carrier mobility for both NMOS, Rim et al., Strained Si NMOSFETs for High Performance CMOS Technology, 2001 Symposium on VLSI Technology Digest of Technical Papers, p. 59, IEEE 2001; and PMOS, Nayak et al., High-Mobility Strained-Si PMOSFET's IEEE Transactions on Electron Devices, Vol. 43, 1709 (1996), devices. The current state-of-the-art for producing a high-quality, relaxed Si1-xGex buffer layer requires the growth of a several μm thick compositionally graded layer, Rim et al.; Nayak et al., supra. However, the density of threading dislocations in these SiGe layers is still high, typically >106/cm2, and the layer growth process is expensive and time consuming. Alternate methods to efficiently relax thinner, e.g., 100 nm to 500 nm, strained SiGe layers on silicon have been implemented, although not all of these methods have been commercialized. In particular, hydrogen, in the form of H+ or H2+ implantation, or He+ implantation, followed by an appropriate anneal, has been used to increase the degree of SiGe relaxation and to reduce the density of threading dislocations; Mantl et al., Strain relaxation of Epitaxial SiGe layers on Si (100) improved by hydrogen implantation, Nuclear Instruments and Methods in Physics Research B 147, 29, (1999); Mantl et al., Thin strain relaxed SiGe buffer layers on Si and SOI made by He+ ion implantation and annealing, Proceedings of the Third International Conference on SiGe(C) Epitaxy and Heterostructures (ICSI3), p. 120 (2003); and Lee et al., Strained-Si N— and PMOSFETs on Thin Graded SiGe Virtual Substrates, Proceedings of the Third International Conference on SiGe(C) Epitaxy and Heterostructures (ICSI3), p. 135 (2003).
The goal of all of these techniques is to produce a “virtual substrate” having a lattice constant parallel to the virtual substrate surface which is larger than that of bulk silicon. On this surface, a thin layer of epitaxial silicon may be grown, which is under tensile strain parallel to the surface. Because of the changes in band structure from the tensile strain, both electron and hole mobilities are significantly enhanced. Germanium is used in the virtual substrate because it has a larger lattice constant than silicon and is fully miscible, i.e., 100% bulk solid solubility, with silicon at all concentrations, meaning that it tends to stay in the substitutional sites in the silicon lattice. However, the production of these virtual substrates is expensive and invariably results in a high density of threading dislocations and other defects.
Instead of adding a larger atom, such as germanium, to the silicon substrate, an alternative method to producing tensile strain in a surface layer of silicon has been to add a smaller atom, such as carbon, to the surface, Eberl et al., U.S. Pat. No. 5,986,287, granted Nov. 16, 1999 for Semiconductor structure for a transistor. This requires growing a very thin layer of Si1-yCy on top of a conventional silicon substrate. Only about 1% carbon, e.g., y=0.01, is needed to achieve significant tensile strain. This method, once commercialized, may be significantly cheaper than the virtual substrate method using relaxed SiGe, and will produce similar gains in electron and hole mobilities. However, a problem arises because carbon is not very miscible in silicon. The bulk solid solubility is only ˜10−5, because of the large mismatch in bond length and energy between silicon and carbon. At higher concentrations, the carbon leaves silicon substitutional sites and forms various clusters. However, because of surface mechanisms, Mitchell et al., Substitutional carbon incorporation in epitaxial Si1-yCy layers grown by chemical vapor deposition, Appl. Phys. Lett., 71, 1688 (1997), it has been found that tensile Si1-yCy films, with the carbon mostly in substitutional sites, may be fabricated provided low deposition temperatures are used, e.g., between 400° C. and 650°. Higher temperature processing of these films, to form transistors, such as nMOSFETs or pMOSFETs, results in carbon clustering, and the resulting electron and hole mobilities are generally poor.
Clearly, a means to stabilize the carbon in the silicon lattice at higher processing temperatures, e.g., 1000° C., will be extremely valuable. Specifically, tensile strained Si1-yCy layers have been used to produce pMOSFETs with enhanced performance by low temperature processing, e.g., ˜750° C. or lower, Quinones et al., Enhanced Mobility PMOSFET's Using Tensile-Strained Si1-yCy Layers, IEEE El. Dev. Lett., 20, 338 (1999). ID enhancements of 12-28% for 15 nm thick layers, where y=0.005, have been reported. Samples where y=0.01 exhibited poor performance. Surface channel tensile strained Si1-yCy layers have been used for nMOSFETs, but without improved mobilities at room temperature, Rim et al., Characteristics of Surface-Channel Strained Si1-yCy n-MOSFETs, Mat. Res. Soc. Symp. Proc. vol. 533, 43 (1998), even though process temperatures were limited to 600° C. Carbon in interstitial sites and in clusters may have contributed to this lack of improvement.
In addition to surface channel tensile Si1-yCy layers, carbon doping may be used in other device structures as well. Adding carbon to compressive SiGe layers, thereby forming a compressive SiGeC layer, has been shown to improve buried p-channel MOSFETs: Quinones et al., Design, Fabrication, and Analysis of SiGeC Heterojunction PMOSFETs, IEEE Trans. El. Dev. 47, 1715 (2000); Mocuta et al., Si1-x-yGexCy-Channel P-MOSFET's with Improved Thermal Stability, IEEE Elec. Dev. Lett., 21, 292 (2000). A dual channel architecture may be desirable: wherein a buried, compressive SiGe channel is used for pMOS and a surface, tensile SiC channel is used for nMOS, because tensile SiC has large conduction band offset, e.g., ˜65 meV per atomic % carbon, but has a small valence band offset, while compressive SiGe has large valence band offset and a small conduction band offset, Rim et al., Metal-oxide-semiconductors capacitance-voltage characteristics and band offsets for Si1-yCy/Si heterostructures, Appl. Phys. Lett., 72, 2286 (1998).
Adding carbon to silicon or SiGe provides advantages for heterojunction bipolar transistor (HBT) structures, U.S. Pat. No. 5,986,287, particularly in the case of compressive Si1-x-yGexCy base layers. The purpose of the carbon in this case is to reduce the outdiffusion of boron dopants in the base, allowing thinner base layers and faster HBT devices Sturm, Advanced Column-IV Epitaxial Materials for Silicon-Based Optoelectronics, MRS Bulletin, April 1998, pp. 60-64. Carbon may also be used to limit boron diffusion in source/drains and source/drain extensions Carroll et al., Complete suppression of boron transient-enhanced diffusion and oxidation-enhanced diffusion in silicon using localized substitutional carbon incorporation, Appl. Phys. Lett., 73, 3695 (1998). Finally, Si/SiGe/SiGeC multilayers may also be used in various combinations to produce quantum wells for optical devices Sturm, supra; Houghton et al., Band Alignment in Si1-yCy/Si (001) and Si1-xGex/Si1-y/Si (001) Quantum Wells by Photoluminescence under Applied [100] and [110] Uniaxial Stress, Phys. Rev. Lett., 78, 2441 (1997).
These carbon-doped films have been fabricated using typical silicon precursors, e.g., SiH4, SiH2Cl2, Si2H6, germanium precursors, e.g., GeH4, and carbon gas precursors, such as C3H8, C2H4, CH4, tetrasilylmethane C(SiH3)4, which has no C—C or C—H bonds, C(SiH2Cl)4, methylsilane CH3SiH3, dimethylsilane (CH3)2SiH2, trimethylsilane (CH3)3SiH, tetramethylsilane Si(CH3)4, and tetra-ethylsilane Si(CH3CH2)4, Mitchell et al., supra; Mi et al., High quality Si1-x-yGexCy epitaxial layers grown on (100) Si by rapid thermal chemical vapor deposition using methylsilane, Appl. Phys. Lett., 67, 259 (1995); Foo et al., Si1-yCy/Si (001) gas-source molecular beam epitaxy from Si2H6 and CH3SiH3: Surface reaction paths and growth kinetics, J. Appl. Phys. 93, 3944 (2003); and Chandrasekhar et al., Strategies for the synthesis of highly concentrated Si1-yCy diamond-structured systems, Appl. Phys. Lett. 72, 2117 (1998), and references therein. Precursors with no C—C bonds and fewer C—H bonds are preferred, because these bonds require high decomposition temperatures. In practice, methylsilane has been the most popular choice.
In spite of the problems seen thus far with the thermal stability of these layers, in light of the low bulk solubility of carbon in silicon it is remarkable that they are as stable as they are. The key seems to be a stabilization mechanism as a result of the surface structure formed during film growth. For example, theoretical calculations indicate there are sub-surface sites of the silicon (001) 2×1 reconstructed surface under compressive stress, which would favor carbon substitution, with nearby sites under tension; Liu et al., Ab initio investigation of C incorporation mechanisms on Si(001), Appl. Phys. Lett. 76, 885 (2000); Remediakis et al., Thermodynamics of C Incorporation on Si(100) from ab initio Calculations, Phys Rev. Lett. 86, 4556 (2001). At low growth temperatures and high growth rates, the carbon may be “frozen in” at these sites, increasing the proportion of substitutional carbon Mitchell et al., supra. It may be expected that the nearby tensile sites favor germanium incorporation, which in turn should further stabilize the carbon. In addition, it is well known that adding even a small amount of germanium greatly enhances the film growth rate, which in turn should enhance the “freezing in” effect.
A similar surface growth stabilization mechanism has been found for a silicon (111) surface, involving germanium and boron, an atom similar in size to carbon; Tweet et al., Direct Observations of Ge and Si Ordering at the Si/B/GexSi1-x(111) Interace by Anomalous X-Ray Diffraction, Phys. Rev. Lett. 69, 2236 (1992); Tweet et al., Increased thermal stability due to addition of Ge in B/Si(111) heterstructures, Physica B 221, 218 (1996). The position of boron and germanium were determined by compressive and tensile sites, respectively, as controlled by the surface reconstruction. Low temperature growth was required to “freeze in” the structure. It was found that germanium enhanced the thermal stability of the boron by at least 100° C. In the case of the silicon (100) surface, it has been shown that adding 0.2% of carbon to a SiGe alloy with 10% germanium increases the thermal stability by 250° C.; Mocuta et al., Carbon incorporation in SiGeC alloys grown by ultrahigh vacuum chemical vapor deposition, J. Vac. Sci. Technol. A 17, 1239 (1999).